1. Field of the Invention
The present invention relates to a method for treatment of arsenic-contaminated soil, said method being designed to insolubilize arsenic in soil, thereby reducing the amount of leachable arsenic below the level specified by the environmental standard value for soil.
2. Description of the Related Art
Factories and establishments handling chemicals and pesticides often cause soil contamination with heavy metals discarded or leaking out of equipment and apparatus. One conventional way to address this problem was by removal of contaminants from soil by heating or washing, or by insolubilization of contaminants with chemicals added to soil. Treatment of soil contaminated with organoarsenic compounds originating from chemical weapons involves heating of contaminated soil in a kiln or the like for decomposition of contaminants. However, there has been no effective way to make arsenic compounds harmless because they assume various forms, such as trivalent or pentavalent inorganic or organic arsenic compounds which vary in characteristic properties.
Conventional technologies involving washing are disclosed in the following literature.    I. A. Legiec, L. P. Grifin, P. D. Walling, Jr., T. C. Breske, M. S. Angelo, R. S. Isaacson, M. B. Lanza, “DuPont Soil Washing Technology Program and Treatment of Arsenic Contaminated Soils”, Environmental Progress, 16(1), 29-34 (1997).
This literature is concerned with washing with a cleaning agent, which is a strong alkaline aqueous solution of sodium hydroxide.    V. Hornburg, B. Luer, “Comparison between Total- and Aqua Regia Extractable Contents of Elements in Natural Soils and Sediments”, J. Plant Nitr. Soil Sci. 162, 131-137 (1999).
This literature is concerned with washing with a strong acidic and strong oxidizing acid.
These conventional technologies involving strong alkaline or acidic chemicals suffer the disadvantage of requiring post-treatment to neutralize washed soils. Another disadvantage is difficulties with disposal of waste liquids and necessity for careful handling and safe storage of chemicals.
A method for decontamination by evaporation with heating is disclosed in Japanese Patent Laid-open No. 2004-50029. This method consists of mixing arsenic-contaminated soil with a silicone oil and heating the mixture at 600° C. or above, preferably 700-1000° C., in an inert gas atmosphere, so that heating decomposes the silicone oil to give silicon (Si). The thus released silicon reduces heavy metal compounds in contaminated soil, thereby forming heavy metals (in the form of simple substance or oxide), and the resulting heavy metals are evaporated and removed by heating. This method suffers the disadvantage of requiring a heating furnace that runs while keeping its inside free of oxygen. Such a heating furnace costs too much for treatment of contaminated soil in large quantities.
There is another conventional technology based on a combination of heating and washing, as disclosed in Japanese Patent Laid-open No. 2002-177941. It is designed to effectively remove arsenic (regardless of its form) from contaminated soil. The object is achieved by heating arsenic-contaminated soil and subsequent washing or by washing arsenic-contaminated soil and subsequent heating. This technology relies on the fact that trivalent arsenic compounds are volatile (at 465° C. for As2O3) but hardly soluble in water and hence easy to remove by heating, and pentavalent arsenic compounds are soluble in water (with a solubility of 414 g/L at 30° C. for As2O5) but hardly volatile and hence easy to remove by washing.
Unfortunately, trivalent and pentavalent arsenic compounds greatly vary in properties (volatility and water solubility) from elemental arsenic when they are contained in or adsorbed by soil. Moreover, it is difficult to predict the form of arsenic in arsenic-contaminated soil by analysis. Therefore, it is difficult to determine what to do first—heating for trivalent arsenic compounds or washing for pentavalent arsenic compounds.
The foregoing will be understood from Table 1 below which shows how arsenic is removed in different ratios when arsenic-contaminated soil is washed with water (ten times the weight of soil) with or without previous heating. It is noted from Table 1 that washing without previous heating removes only 6 to 15% of arsenic from contaminated soil. On the other hand, heating arsenic-contaminated soil at 550° C. for 20 minutes in air is not so effective as to reduce the content of arsenic. Moreover, heating and ensuing water washing are only slightly effective in removal of arsenic.
TABLE 1Ratio ofremoval ofContent ofarsenicContent ofarsenic infromarsenic inRatio of removalarsenic-contaminatedsoil afterof arsenic bycontaminatedsoil byheatingwashing fromNo.soil (mg/kg)water washing(mg/kg)heat-treated soil13015%271%244 9%381%318 6%232%41010%171%51110%191%
Soil usually contains divalent or multivalent metals, such as calcium. These metals form salts with pentavalent arsenic ions and resulting salts are only slightly soluble in soil water in soil. For example, calcium arsenate Ca3(AsO4)2, which is a pentavalent arsenic compound, has such a small solubility product (6.8×10−19) that its removal by water washing is essentially impossible.
A method for insolubilization is disclosed in Japanese Patent Laid-open No. 2003-290759. This method consists of adding cement, hydroxyapatite, and poly(ferric sulfate) to contaminated soil containing heavy metals such as arsenic, thereby immobilizing heavy metals and preventing their leaching. Treatment by this method is economically disadvantageous because it employs expensive chemicals.
Organoarsenic compounds originating from chemical weapons are usually vaporized or decomposed by heating at 300 to 500° C. By contrast, when contained in soil, they react with metals (such as calcium, iron, and magnesium) in soil at the time of heating to convert into compounds which are hardly volatile and very difficult to remove by washing.
Table 2 below shows the test results obtained by heating a sample soil artificially contaminated with diphenyl-arsinic acid (DPAA: C12H11AsO2), which is one of organo-arsenic compounds. It is noted that after heat treatment at 550° C. for 20 minutes, DPAA mostly (more than 99.93%) disappears but largely changes into inorganic arsenic compounds in soil. Thus heat treatment removes only 30 to 34% of arsenic.
TABLE 2Amount ofAmount ofNo.contaminant addedremaining contaminant1In the form of DPAA:In the form of DPAA:500 mg/kg<0.1 mg/kg (<0.02%)In the form of As:In the form of As:143 mg/kg100 mg/kg (69.9%)2In the form of DPAA:In the form of DPAA:1000 mg/kg7 mg/kg (0.7%)In the form of As:In the form of As:2863 mg/kg1900 mg/kg (66.4%)Patent document 1: Japanese Patent Laid-open No. 2004-50029 (Paragraphs 0014 to 0015)Patent document 2: Japanese Patent Laid-open No. 2002-177941 (Paragraphs 0010 to 0011)Patent document 3: Japanese Patent Laid-open No. 2003-290759 (Paragraphs 0009 to 0016)